Please tell me you live on a farm/ranch and that is why you want a roaming cat. As the recipient of neighborhood roaming cats, I beg you not to allow your cat to roam if you live in a neighborhood. In two different neighborhoods I have lived, I have had cat prints on my newly washed/waxed car and cat poop in the yard. I also had one live under my porch except to go home and eat.
I have had both kittens and a cat. I took the cat back. Even though it was fixed it still sprayed, in fact it sprayed me while sitting in bed. It was also cantankerous, not at all what I wanted. Yes, the extra expenses w/ a kitten is costly, but the potty training isn't too bad. And, it will grow up with the dog around, as you already mentioned. You could get a older kitten that already has it's shots/fixed, but they may not be as available.